The detection of moving objects through an obscuring media can be a difficult task, even for advanced electronic imaging systems. Image acquisition by electronic systems is most readily accomplished when the moving object is located in a transparent medium. However, even transparent media can contain sources of signal aberrations that make image acquisition difficult. Obscuring media are media that impede image acquisition due to phenomena such as absorption, scattering, aberration, or boundary effects.
For purposes of detecting and tracking moving objects, electronic imaging systems are generally preferred over other means of image acquisition for several reasons. Electronic imaging systems are not only able to generate images, but also accurately determine one or more of the position, range, bearing and speed of a moving object. Thus electronic systems can track as well as identify the moving object. Another advantage of electronic imaging systems is the ability of electronic systems to digitize the image signal of the object and perform computational processing tasks such as contrast enhancement, smoothing and blending of adjacent image pixels, deconvolution, and subtraction of one image from another.
Electronic imaging systems may not effectively image or track a moving object if the object is moving in an obscuring medium. Certain media obscure the object, that is, make detection difficult. Moving objects located below the ocean surface can be difficult to detect by surface ships and submarines due to natural scatterers, absorbers, and non-uniformities present in sea water and relevant interfaces. Such interfaces may include the surfaces of the moving object or a moving detection system. The atmosphere and relevant boundaries may each be obscuring as well. For electromagnetic radiation that is not highly absorbed or scattered by the atmosphere, obscuration may derive from turbulence. Turbulence can result from natural conditions in the medium as well as from local turbulence introduced by the motion of a moving object or a moving detection system such as would be found, for example, on a torpedo or high speed missile. Natural or man-made turbulence produces refractive index gradients that have the effect of introducing aberrations into the image propagation path and thereby produce distorted and sometimes ambiguous images. Turbulence is most likely to occur near a large area interface between the atmosphere and either land or a body of water such as the sea. For example, mirage images may arise due to intense solar heating of a surface such as a macadam roadway. Heat rising from the roadway heats the air near the interface (boundary layer) to a temperature that exceeds the temperature of the air located several meters above the surface. A refractive index gradient in the terrestrial boundary layer is created that reflects a portion of the incoming solar radiation. The effect appears similar to that of the reflection by a small body of water.
Another area where atmospheric turbulence plays an important role in obscuring images is in the marine boundary layer. The marine boundary layer is the vertical column of air immediately adjacent to the ocean surface. Since ocean water temperatures during the day are typically cooler than air temperatures, the air in the boundary layer is cooled relative to the air several meters above the water. As a result, images of objects located within the marine boundary layer are distorted.
Because of the imaging difficulties associated with objects located near these types of atmospheric interfaces, a moving object is more difficult to detect if it moves close to the surface. This is the case for cruise and sea-skimming missiles for example, which fly at high speed just several feet above the land or sea surface to defeat detection. Such surface-hugging, moving objects are difficult to detect through the turbulent atmosphere. In some cases, observations from satellites or reconnaissance aircraft can detect these objects. The success of such imaging systems is based on the observation axis used by the imaging system relative to the marine or terrestrial boundary layer. That is, the path through the aberration boundary layer is shorter for detection from above the layer than for detectors located at the surface. The latter detectors must collect image data through a longer path length in the boundary layer.
However, imaging from above the boundary layer does not correct for the problems of imaging through an aberrational atmosphere. Further, reliance by a platform located at the surface on an airborne system to provide image and tracking information has numerous inherent risks. One is the potential for unreliable or untimely communication between the vertical platform and the surface platform. Another risk is based on the requirement that a vertical platform be available to scan the region surrounding the surface platform when needed.
Thus, it may be appreciated that there is a need for a system that provides accurate imaging of objects through an obscuring medium, and for tracking moving objects through an obscuring medium even when the imaging platform itself is located within the obscuring medium.